I don't think there is any advantage to overclock the mem. I would leave it the same or lower it anyway. OS shouldn't matter really, whatever is your personal preference is IMHO. If you do switch to Windows there are reports of some having difficulties getting that 4th card to show up. What mobo are you using?
my motherboard is MSI 890FXA-GD70.
the memory is not a problem, on 6990 you cannot downclock memory more then 100mhz under core speed (example: gpu 900 memory 800, gpu 970 memory 870). i think my problem is low voltage, on ubuntu amdoverdrivectrl the max voltage is 1.175.